The present invention is directed to pressure-sensitive hot-melt adhesives which can be applied to flexible substrates to form adhesive tapes, labels, wall coverings, bumper stickers or any other application where a pressure-sensitive adhesive is needed.
Pressure-sensitive adhesives are well-known in the art. The method of applying these adhesives leaves much to be desired, however, because the adhesives are usually applied using a solvent carrier. The solvent carrier after application is evaporated into the atmosphere creating an air pollution problem. Superimposed upon the air pollution problem was the problem that the solvents used in the application of pessure-sensitive adhesives were normally the coal and the petroleum derivatives including aliphatic and aromatic hydrocarbons. The cutting down of lead in automotive fuels forced gasoline manufacturers to switch to the use of aromatic hydrocarbons to increase the octane ratings of their gasolines. This resulted in a critical shortage of aromatic hydrocarbons for the pressure-sensitive adhesive tape manufacturers. The shortage of petroleum feed stock also resulted in shortages of other forms of solvent.
The art in the area of pressure-sensitive adhesives most closely related to the present invention includes:
Patent No. Inventor Issue Date ______________________________________ U.S. 3,239,478 Harlan 1966 U.S. 3,625,752 Korpman 1971 U.S. 3,676,202 Korpman 1972 U.S. 3,783,072 Korpman 1973 U.S. 2,784,587 Chambers 1973 S. African 700,881 Goodyear Tire ______________________________________
And various mimeographed materials authored by Petershagen and others of Shell Oil Company. There is also a publication in Adhesives Age of December, 1972, pages 30-33 entitled "Thermoplastic Rubber and Hot-Melt and Solution Adhesives" bby Harlan, Luter, Petershagen and Robertson.
The Korpman patents mentioned, except for U.S. Pat. No. 3,783,072, are based upon solvent systems resulting in the evaporation of large amounts of aromatic solvents into the atmosphere resulting in both air pollution and comsumption of vitally needed automotive fuel ingredients. Another disadvantage of the solvent based adhesive systems is that they require huge drying ovens for the removal of solvent which consume appreciable amounts of energy and take up large amounts of floor space, much more in fact than apparatus involved in the actual coating of the substrate with the adhesive. On the other hand, the solventless adhesives mentioned in his U.S. Pat. No. 3,783,072 patent are all in viscosity range of 750,000 - 2,000,000 centipoises which is beyond the coating viscosity range by conventional hot-melt technique. The molten and free-flowing adhesive of our invention is able to be coated onto a web by conventional hot-melt coating devices such as a transfer roll coater, slot coater, two-roll coater, etc. The mimeographed materials and publication of Petershagen mentioned above, while disclosing broad general classes of ingredients which can be incorporated into hot-melt pressure-sensitive adhesives, do not specifically pin point the combinations or ratios of ingredients which provide the vitally balanced tape properties, such as, Panel Adhesion, 20.degree. hold, quick-stick and adhesive strength required by the adhesive tape industry.